1. Field of the Invention
The present invention relates to a display using a current driven type light emitting element as a pixel, and to a method for manufacturing the same.
2. Description of the Related Art
In recent years, flat panel displays are used in many fields and in many places and, as computerization develops, the importance of them is increasingly enhanced.
While, when we say a representative of the flat panel displays, it is a liquid crystal display (hereinafter referred to simply as “LCD”). As flat panel displays based on a displaying principle that is different from that of the LCD, development of organic electroluminescent (hereinafter the “electroluminescent” is abbreviated as “EL”) display, inorganic EL display, plasma display, light emitting diode display, fluorescent display tube, field emitting type display, etc. is also active.
Of the above mentioned displays, the each display other than the LCD are the ones called “spontaneous light emitting type displays” and they greatly differ from the LCD's that are called “light receiving type displays” in respect of the following points (1) to (4).
(1) Power Consumption
The individual pixels in the LCD do not emit any light themselves, and function as light shutters that transmit and shield light. Therefore, the LCD's other than the transmitting type LCD's need backlights. When displaying, regardless of the mode of the displayed information, the backlights are always needed to be on. For this reason, when displaying, power as much as the full displaying state is consumed on steady basis.
In contrast, in the spontaneous light emitting type, the individual pixels themselves emit light. Therefore, there is no need to provide a separate light source and, only certain pixels are needed to be turned on, depending on the mode of the displayed information. Therefore, the spontaneous light emitting type needs less power than the light receiving type displays.
(2) Contrast
In case of the LCD, the individual pixels cut off the light from the backlight or the environmental illumination light to obtain a state of darkness. However, it is difficult to cut off all the light leakage from the individual pixels completely, as a result, improving the contrast is difficult.
In contrast, in the spontaneous light emitting type displays, a state where the individual pixels do not emit any light is a state of darkness. Therefore, it is possible to obtain an ideal state of darkness easily, so that it is easy to improve the contrast.
(3) Viewing Angle Dependence
In the LCD, because the liquid crystal molecule has a double refractivity, and transmission and cutting off of the light is controlled by controlling the arranging direction of the liquid crystal molecule, the LCD has viewing angle dependence. Therefore, in case of the LCD, the state of display changes, depending on the viewing direction. In contrast, in the spontaneous light emitting type display, such problem hardly occurs.
(4) Response Characteristic
The LCD controls the arranging direction of the liquid crystal molecule by utilizing the dielectric constant anisotropy of the liquid crystal molecule. Therefore, from the viewpoint of principle, the time of response to the electric signal becomes 1 millisecond or more. For this reason, when displaying motion pictures, an after image may occur.
In contrast, in the spontaneous light emitting type display, the on/off of the pixel is controlled by utilizing the carrier transition, electron emission, plasma discharge, etc. Therefore, the response time to the electric signal is as short as the nanosecond level and, therefore, displaying motion pictures do not cause the occurrence of an afterimage.
Even among the spontaneous light emitting type displays having the above mentioned advantages, the organic EL display which uses the organic EL element (also called “the organic light-emitting diode”) as the pixel has a practically extremely excellent feature that the driving (light emitting) voltage is low. Actually, the light emitting starting voltage of the organic EL element is 10 V or less, namely, compared to the light emitting element that is used in other spontaneous light emitting type displays, the driving voltage is extremely low. The organic EL display, having a feature that low voltage driving is possible, is suitable as, for example, a display for a battery driven type portable electronic device which the upper limit of power source voltage is limited. For this reason, nowadays, investigations of the organic EL displays have vigorously been made.
The basic structure of the organic EL element is the structure in which a light emitting layer formed from organic material is sandwiched in between an anode and a cathode. As necessity, in between the anode and the light emitting layer, a hole injection layer or hole transportation layer is formed, while, in between the cathode and the light emitting layer, an electron injection layer or electron transportation layer is formed. Also, the color of light emission is controlled by doping fluorescent dye or the like on the light emitting layer.
Here, in this specification, in a current driven type light emitting element, all layers that are formed in between the cathode and the anode are named generically as “light emitting portion”. Also, in a case where the current driven type light emitting element is an organic EL element, that light emitting portion is referred to as “organic EL layer” in particular. The organic EL layer includes the hole injection layer, hole transportation layer, light emitting layer, electron transportation layer, and electron injection layer.
FIGS. 6 and 7 schematically illustrate sectional structures of the organic EL element. An organic EL element 30 illustrated in FIG. 6 is manufactured by forming a first electrode layer 33 on an electrically insulative substrate 1 having light transmittance such as a glass substrate or a plastic substrate, and forming an organic EL layer 35 and second electrode layer 37 sequentially thereon.
This organic EL element 30 is of a type in which the light emission El from the organic EL layer 35 is taken out from the electrically insulative substrate 1 side and is sometimes called “a bottom emitting type”. In general, the first electrode layer 33 is used as an anode and the second electrode layer 37 is used as a cathode. The second electrode layer 37 is a non-light transmitting electrode formed of metal, in many cases.
An organic EL element 40 illustrated in FIG. 7 is manufactured by forming an electrode 43 on the electrically insulative substrate 1, and forming an organic EL layer 45 and a light transmitting second electrode layer 47 sequentially thereon.
This organic EL element 40 is of a type in which the light emission E2 from the organic EL layer 45 is taken out from the side opposite of the electrically insulative substrate 1, and in some cases is called “a top emitting type”. Preferably, at least one of the electrically insulative substrate 1 or electrode 43 is non-light transmitting. In many cases, the electrode 43 is a non-light transmitting electrode that is formed of metal.
As the organic EL layers 35 and 45, in a case where low molecular weight material is used as the material of organic EL layers 35 and 45, the layers are generally formed by a vacuum deposition method. In a case where high molecular weight material is used, the layer is formed by a spin-coating method, printing method, or transfer method, by making the material into a solution. In a case where a large number of the organic EL layers 35 or 45 are formed at one time as when forming the pixels of the display, when low molecular weight material is used as a material of the individual organic EL layer 35 or 45, a mask deposition method for example is applied when high molecular weight material is used, an ink jet method, printing method, or transfer method is applied. In recent years, the availability of a coatable low molecular weight material has been reported.
In the organic EL elements 30 and 40, light is emitted by applying voltage in between the electrodes (between the anode and the cathode) and therefore current flow is applied through the organic EL layer 35 or 45. Conventionally, only fluorescence emission, which occurs when returning from a singlet excited state to a ground state, was utilized. As a result of recent studies, phosphorescence emission which occurs when returning from a triplet excited state to a ground state can be utilized. Thereby, the light emitting efficiency is improved.
Although not illustrated in FIGS. 6 and 7, in general, since the properties of the organic EL element 30 and 40 are significantly deteriorated by water or oxygen, the reliability is ensured by so-called “sealing”. Sealing of the organic EL element 30 or 40 is performed, for example, by forming a space, in which the organic EL element 30 or 40 is accommodated, by using the electrically insulative substrate 1 and another substrate such that the organic EL element 30 or 40 does not contact to water or oxygen, and filling an inert gas into the space, or by covering the organic EL element 30 or 40 with a deposited thin film.
By using the organic EL elements 30 or 40 as the pixels, it is possible to obtain a full color display. The method for full colorizing includes: a three-color juxtaposition method of precisely disposing each organic EL element whose light emitting colors are red, green, and blue, per each pixel of the display; a CF method of combining the organic EL element, whose light emitting color is white, with primary-color (red, green, and blue) color filters (CF); and a CCM (Color Changing Medium) method of combining the organic EL element, whose light emitting color is blue, with red and green fluorescence converting dye filters.
The display which uses the organic EL element 30 or 40 as the pixel can roughly be classified, according to their driving methods, into a passive matrix type and an active matrix type, as in the case of the LCD.
In the passive-matrix type display, when disposing a large number of the organic EL elements 30 or organic EL elements 40 in the a matrix form, one piece of the first electrode layer 33 or electrode 43 is formed, for example, per each element row. Also, one piece of the second electrode layer 37 or 47 is formed per each element column. Further, at each of the intersection parts of the electrodes, as viewed from above, the organic EL layer 35 or 45, sandwiched by the first electrode layer 33 or electrode 43 and the second electrode layer 37 or 47, is formed.
This type of display has a merit that the structure is simple. However, for displaying an image by a time-divisional scan, the type has a demerit that, compared to the active matrix type display, it is necessary to increase the instantaneous brightness of the individual organic EL element 30 or 40 by the by multiple of the number of the scanning lines. For example, for obtaining the display whose resolution is VGA (Video Graphics Array) or more, the instantaneous brightness of the individual organic EL element 30 or 40 is desired to be 1000 cd/m2 or more.
On the other hand, in the active matrix type display, for example, one first electrode layer 33 or electrode 43 is formed per each individual organic EL element 30 or 40, and one large-sized electrically conductive film is formed, as the second electrode layer 37 or 47, common to all the organic EL elements 30 or 40. Also, for controlling the operations of the organic EL elements 30 or 40, one switching circuit portion is disposed per each individual organic EL element 30 or 40. Each switching circuit portion is constituted of, for example, a plurality of semiconductor switching elements, and controls the operation of the organic EL element 30 or 40 corresponding to the switching circuit part, according to the pixel selecting signal supplied from the scanning line disposed per element row and to the image signal supplied from the data line disposed per element column.
Although this type of display has complex structure, compared to the passive-matrix type display, it has a lot of merits such as that the brightness of the light emitting from the individual organic EL element 30 or 40 is not needed to be very high, that the power consumption can be suppressed, and that the crosstalk between the pixels is unlikely to occur.
Further, by using a polycrystalline silicon (polysilicon) film or continuous grain boundary silicon (CG silicon) film for forming the semiconductor switching element, since the charge mobility is higher in those films than in the amorphous silicon film, forming of a high operating speed switching element is possible. As a result of this, it becomes easy to form various kinds of control circuits on a single substrate together with the pixels, thereby making the display small in size, low in cost, multi-functional, etc. Also, since the semiconductor switching element formed by using a polycrystalline film or continuous grain-boundary silicon film enables processing of a large current, it is suitable for controlling driving of the organic EL element which is a current driven type element.
When constituting the pixels in the active matrix type display by the top emitting type organic EL element 40, since the light emitting area rate is not limited by the circuit components such as the switching circuit portion, the bus lines, etc., more multi-functional and more complex circuits become easy to be form on the electrically insulative substrate 1.
FIG. 8 illustrates a constitution example of the pixel circuit in the active matrix type display 100 which uses the organic EL elements 30 as the pixels.
As illustrated in the figure, in the display 100, a large number of the organic EL elements 30 are disposed in the a matrix form. One piece of scanning line 11 is disposed per each element row along the element row, while one piece of data line 13 and power source supply line 17 are disposed per each element column along the element column.
In the actual display 100, the scanning line 11 is located, when viewed from above, in between two adjacent element rows, while the data line 13 and power source supply line 17 are located, when viewed from above, in between two adjacent element columns.
One switching circuit portion 20 per each individual organic EL element 30 is connected. The illustrated switching circuit portion 20 is constituted by the first thin film transistor 21, second thin film transistor 25, and gate retention capacitor 29 connected to the first thin film transistor 21 and second thin film transistor 25. The gate retention capacitor 29 is also connected to the power source supply line 17.
The gate of the first thin film transistor 21 and the scanning line 11 corresponding to the switching circuit portion 20 including the first thin film transistor 21 are electrically connected to each other. The source of the first thin film transistor 21 is electrically connected to the data line 13 and the drain thereof is electrically connected to the gate retention capacitor 29. Also, the gate of the second thin film transistor 25 is electrically connected to a wiring 23d which connects the first thin film transistor 21 corresponding to the second thin film transistor 25 and the gate retention capacitor 29, while the source of the second thin film transistor 25 is electrically connected to the power source supply line 17 and the drain thereof is electrically connected to the organic EL element 30.
When supplying a pixel selecting signal to prescribed scanning lines 11, the gate of each first thin film transistor 21 connected to the scanning lines 11 becomes open. Also, when supplying an image signal to prescribed data lines 13, the gate of the second thin film transistor 25, corresponding to the first thin film transistor 21 whose gate was opened by the above mentioned pixel selecting signal, becomes analogously open depending on the level of the above mentioned image signal. The opening degree of the gate of the second thin film transistor 25 is maintained by the gate retention capacitor 29 corresponding to the second thin film transistor 25. In that state, when voltage is applied to the power source supply line 17, to the organic EL element 30 connected to the second thin film transistor 25 whose gate is open, a value of current which corresponds the opening degree of the gate of the second thin film transistor 25 flows from the power source supply line 17. As a result, the organic EL element 30 emits light. That is, the organic EL element 30 emits light depending on the level of the above mentioned image signal.
Since the display 100 of the above mentioned constitution is an active matrix type, it has the above mentioned various merits. However, since in the display 100, the organic EL element is a current driven type light emitting element and the light emitting efficiency of the organic EL element is not very high, the driving thereof becomes difficult, when the definition is made higher, or the size is made larger.
While, as described above, the organic EL element can be driven with a low voltage, this means that if the light emitting efficiency is the same, a larger amount of driving current is necessary. The efficiency of the display can be expressed as the light emitting efficiency shown by the following formula.
The light emitting efficiency (lm/W)=brightness (cd/m2)·π/[(voltage (V)·current density (A/m2))
The display is demanded, in general, that it has a light emitting efficiency of 2 lm/w or more. For obtaining that light emitting efficiency in the organic EL display, it is necessary to obtain a brightness of approximately 100 cd/m2 or more for each individual organic EL element. To that end, it is necessary that the current density in each organic EL element is 15 A/m2 or more.
Compared to the current density of 1 to 2 A/m2 in the pixel in other spontaneous light emitting type displays such as pixel in an inorganic EL display or pixel in a plasma display, the current density of 15 A/m2 is an extremely high value. Since the LCD is a field driven type, when saying from the viewpoint of principle, there is almost no need to apply a flow of current to the individual pixels.
In the organic EL display, in order to supply a large current as mentioned above to the individual EL element, it is very important to lower the wring resistance of the power source supply line. If the wiring resistance is high, although it is possible to supply a sufficient amount of current to the organic EL element connected to the power source supply line on a near side of the power source supply terminal, it becomes impossible to supply, due to the voltage drop, a sufficient amount of current to the organic EL element connected to the power source supply line on a far side of the power source supply terminal. Resultantly, display unevenness occurs. The greater the number of pixels becomes, or the larger the size of the display screen becomes, the more important it becomes to lower the wiring resistance of the power source supply line.
For instance, Japanese Patent Application Laid-open No. 2002-32037 (reference 1) describes an organic EL display that has enabled supplying a large current to the individual organic EL element by disposing the power source supply line not only in between two adjacent element columns but also in between two adjacent element rows.
Also, Japanese Patent Application Laid-Open No. 2002-40961 (reference 2) describes an organic EL display in which the power source supply lines are collectively taken out around the substrate, while, Japanese Patent Application Laid-Open No. 2002-108262 (reference 3) describes an organic EL display in which the both ends of the power source supply lines are connected to the power source terminal. In the organic EL display described in both reference 2 and reference 3, the wiring resistance of the power source supply line is lowered by making the substantial length of the power source supply line, with respect to the power source, shorter.
However, in all of the organic EL displays described in the reference 1 to 3, in respect of making a higher definition or larger display, the wiring resistance of the power source supply line is relatively high.
As the circuit constitution and driving method of the organic EL display, other than those mentioned above, the apparatus in which the number of the thin film transistors in the individual switching circuit portion are made still larger (refer to “Pixel-Driving Methods for Large-sized Poly-SiAm-OLED Displays” by Yumoto et al, Asia Display/IDW′ 01, P. 1395-1398. ) and the apparatus in which digital gradations are made (refer to “6-bit Digital VGA OLED” by Mizukami, SID′ 00, P. 912-915. and “Full Color Displays Fabricated by Ink-Jet Printing” by Miyashita, Asia Display/IDW′ 01, P. 1399-1402) are known. However, in all of those apparatuses, in respect of making a higher definition or larger displays the wiring resistance of the power source supply line is relatively high.